This invention relates to a halogenated hydrocarbon gas detecting element of cation emission type.
Halogenated hydrocarbon gas has been widely used as refrigerant gas or coolant for electric refrigerators, air conditioners and the like. Halogenated hydrocarbon has a chemical formula wherein one or more of the hydrogen atoms has been replaced by chlorine, fluorine or the like, and typically includes R-12 (CCl.sub.2 F.sub.2), R-22(CHClF.sub.2), etc. These halogenated hydrocarbons are chemically and thermally very stable, harmless to human bodies, and have excellent thermodynamical characteristics when they are used for refrigeration systems.
In the refrigeration systems, the halogenated hydrocarbon gas mentioned above is alternately compressed and expanded to cause its refrigeration actions. A this time, a trace amount of the halogenated hydrocarbon gas may sometimes leak away from compressors, radiators, pipes, etc. If such leaks are left unstopped, efficiency of the refrigeration systems is lowered due to a gradual decrease of the refrigerant gas, with the result of a possible stop of the function of such equipments. Accordingly, severe control is required when such equipments are manufactured in a factory, and it is desirable to periodically check any leak along the pipe systems. Especially, in the refrigeration systems such as air conditioners for cars and the like, there is a greater possibility of leak due to a shock during drive, and a detector which may simply find out the leaks has long been desired.
Generally, the air conditioner as mentioned above is heavy in weight, and in addition it is usually fixed at one place or mounted on a car. Therefore, it is impossible to make an inspections thereof by simply turning or overturning it. Moreover, the leaks are usually too small to find out visually or by a magnifier. Further, it is desirable for an element for detecting leaks to be as miniature as possible since the pipe system, etc. is usually of complicated structure. Still further, the detecting end of an element should preferably be as small as possible in diameter since the detecting element traces the pipe or the like to locate a leak. In addition, it is required that a detecting element can be operated by means of a miniature cell or cells in order to make it easy to handle. While the sensitivity of a detector is desirably as high as possible as a matter of course, it is required for it to detect a gas leak of at least 10.sup.-4 cc/sec (25.degree. C., 1 atm).
There has been proposed a number of halogenated hydrocarbon gas detectors, some of examples of which are being explained below:
A typical detector, a torch, utilizes a kind of flame reaction; it utilizes the phenomenon that the color of flame changes responding to the chemical reaction of the halogen gas mixed into the flame with a copper metal provided in the flame. Although this method is simple, it is often accompanied by errors because the presence or absence of leaks is visually judged. Moreover, the limit of detection according to this method is 10.sup.-2 cc/sec (25.degree. C., 1 atm) at best.
Further, there has been proposed a detector which utilizes high voltage electric discharge. This detector is provided with a pair of electrodes exposed to air with a gap therebetween, to which electrodes is applied a high voltage of several hundred volts for producing an electric discharge at the gap. The discharge stops when halogenated hydrocarbon gas comes into the gap between the electrodes. As a result, the leak can be detected by detecting the change of the discharged current, with the detection limit leveled up to 10.sup.-3 cc/sec (25.degree. C., 1 atm) which is sufficient for practical use. However, this detector, which utilizes the electric discharge, is disadvantageous in for example that the discharge is interrupted due to other external causes such as wind or the like even when there is no leak of the halogenated hydrocarbon gases.
On the other hand, as a detector having sufficiently high sensitivity, there has been known a detector called a cation emission type leak detector. The detector of this type comprises ceramics such as steatite containing Na, K, etc., an ion collector electrode and a heater. The ceramics are heated to a high temperature (e.g. 800.degree. C.), to which ceramics there is provided at a predetermined space an ion collector electrode made of a metal. While a high voltage of about 300 V is applied to the space between the ceramics and the ion collector electrode, the halogenated hydrocarbon gas is reacted on the surface of the ceramics due to the high temperature to emit ions of Na, K, etc. contained in the ceramics, which ions are attracted to and captured by the metallic electrode with the aid of the high voltage. As the result, the leak can be detected by detecting ionic current thus generated. According to this detector, the detection limit is not more than 10.sup.-6 cc/sec (25.degree. C., 1 atm) and thus the detector exhibits very high sensitivity.
However, the detector of this type consumes the electric power of as large as 20.about.30 W because the ceramics must be kept at a high temperature (about 800.degree. C.) as mentioned above, whereby not only a larger size of an apparatus but also a cord for the power source are required. Moreover, when the detector of this type is in contact with an unexpectedly high concentration of the halogenated hydrocarbon gas, the alkaline metals in the vicinity of the surface of the ceramics such as the steatite mentioned above are ionized in large quantitites to form a flow of cations which flows to a cathode undesirably, with the result that the alkaline ions in the vicinity of the surface of said ceramics have been consumed. Accordingly, because of paucity of alkaline ions on the surface of the ceramics, a quick response of the detector can not be expected even if it is brought again into contact with halogenated hydrocarbon gas, and the sufficient detecting sensitivity can not be obtained until the alkaline ions in the ceramics are recovered by having diffused sufficiently in the vicinity of the surface. It is therefore necessary for the ceramics to be heated at a high temperature so that the alkaline ions may readily migrate from the inside of the ceramics to the surface thereof. It is for this reason that the temperature of the element must be kept at 800.degree. C. as aforementioned, but nonetheless it shows unavoidable non-sensitivity during a time of several to several ten minutes after having detected halogenated hydrocarbon gas of a high concentration. Such a high temperature further causes disadvantageously a serious breakage or wear of the metallic electrode, and especially a short life time of a heating wire serving also as an anode. In addition, the detector of this type, which detects the halogenated hydrocarbon gas while consuming the alkaline ions which are finite, has necessarily a limited life time since the whole quantity of the alkaline metal ions contained in the aforesaid ceramics is extremely small.